This invention relates to a process for the low-temperature electrolysis of aluminum, which produces aluminum by electrolysis of aluminum chloride or aluminum bromide.
The electrolytic process for aluminum refining which electrolyzes a bath composed preponderantly of aluminum chloride or aluminum bromide and consequently educes metallic aluminum at the cathode enjoys advantages in that the electrolysis is effectively performed at low temperatures of from 120.degree. to 250.degree. C, the operation is easy, the electrolytic cell is simple in structure, the electric potential applied to the electrolytic bath is low and the current efficiency is high. Nevertheless this process has a disadvantage that electric deposition of aluminum on a cathode plate does not satisfactorily proceed. Because of the disadvantage, this process has so far failed to find utility in practical applications.
Concerning a method whereby molten aluminum is obtained by electrolyzing a molten salt containing aluminum chloride at temperatures higher than the melting point of aluminum (660.degree. C), there have been reported quite a number of studies including a study covering Alcoa Smelting Process ("A Revolutionary Alcoa Process," Business Week, January 20 (1973), J. D. Harper: Eng. Min. J., 174, No. 2 (1973), 30). In contrast, as to a method whereby said electrolysis is effected at temperatures below the melting point of aluminum, there have been reported a relatively small number of studies including Plotnikow et al: Z. Electrochem., 37 (1931), 83, Engelhardt: Handb. Techn. Electrochem., 3 (1934), 384, Czochralski, Mikolajczyk: Wiadomosci. Inst. Metallurg., 2, (1935) 31 and Midorikawa et al: Electrochem., 24 (1956) 562.
Rinzo Midorikawa developed a technique for improving the electric deposition of aluminum on the cathode plate by adding lead chloride to the electrolytic bath (Electrochem., 24 (1956) 562, Japan). However, even when the Midorikawa method is carried out under the most favorable conditions, the current density used in the formation of the electrically deposited layer cannot be raised above 0.5 A/dm.sup.2 and the thickness of the resulting layer is about 0.1 mm at most so that this technique has not been reduced to commercial application.
An object of the present invention is to provide an improvement in a process for the production of aluminum by electrolyzing a bath composed preponderantly of aluminum chloride and/or aluminum bromide for thereby causing electric deposition of aluminum on the cathode plate, which improvement permits production of aluminum of high purity without reference to the nature of the electrically deposited layer.